One of the most important questions to ask when you’re considering solar panels for your home is “how many solar panels do I need?” It’s important because you want to make sure your solar panels can provide enough electricity to offset your power bill and pay back their cost.

Here’s the answer for the average American home:

The average home in the Unites States needs about 9,000 kilowatt-hours of electricity per year. To make that much energy, the home would need twenty-one 320-watt solar panels, for a total installation size of 6.72 kilowatts. That would be enough to produce just over 9,000 kWh in the first year, with the solar panels under warranty for at least 25 years.

That’s the easy answer for the average American house. But your house is anything but average, and many factors go into the calculation that determines what we solar folks call “system size.” To get those variables, you have to answer a few other questions before you can determine a final number of how many panels you need.

Here’s where we tell you the truth: Doing these calculations yourself can provide you with a rough estimate of what you might need, but nothing beats an expert using the most advanced tools that can scan your roof and calculate a very good estimate of your home’s solar potential.

Below, we’ll work through how we got to the numbers we used for an average home. Here’s a step-by-step list of preliminary questions you have to answer before you get to the number that will answer the final question:

Note: What we’re talking about here applies to people who live in their utility’s service area and intend to install a “grid-tied” solar panel system. If you’re interested in off-grid solar, see our guide to finding out how many solar panels you need to go off-grid.

Step 1: Determine your energy usage

Looking at usage over a year, you can get a good picture of your total energy needs. In fact, many utility companies base the total allowable size of your home solar panel system on how much energy you used in the 12-month period prior to installation, so you and your installer need to know this number.

Some utility companies make it simple to find your kWh needs by putting your year-to-date or last 12 months’ usage somewhere on your bill. Others make it more difficult by withholding that information, forcing you to call them, go on their website, or (gasp) find all 12 of your last bills and add up the monthly totals to calculate the annual usage.

Here’s an example from one utility’s online portal that shows monthly and total usage in kWh, along with the average daily temperature, which can illustrate why usage is higher:

The image above shows an annual usage of just over 9,000 kWh, which we’ll use in the next few examples as a benchmark. Like we said above, the average annual usage across the United States is about 9,000, so this number will provide a good estimate for most folks.

Usage goes up and down on a month-by-month basis, and sometimes has weird spikes. Looks like this person left town for July, but maybe hosted guests in September. Those weird spikes are why it’s important to look at your full years’ usage.

Step 2: Determine how many kWh you get per kW of solar

The next big factor in finding out how many solar panels you need is how much electricity you can squeeze outta your panels. For that, you need to know how sunny it is where you’re live and determine the number of kWh each kW of panels can make.

Thankfully, the folks over at the National Renewable Energy Laboratory have made the handy map below to make it easy. check it out:

The map shows different colored areas based on how many kWh you can generate in a year with 1 kW of solar panels in an ideal setting.

So let’s do an example: most of that map lies in the 1,500 to 1,700 range. So let’s use the average: 1,600. How do you feel about St. Louis?

If we need 9,000 kWh in Saint Louis, MO, which is in the “1,600” zone of the map, we’d take that 9,000 divide by 1,600 and get 5.625. That’s the size of our system in kW under ideal conditions. Unfortunately, solar panels don’t exist in ideal conditions. They live on your roof, where heat, wiring, and more can cause slight losses of electricity.

Expect those losses to be about 15% of that ideal number. So take your 5.625 kW and divide it by 85% (0.85). That gives you 6.62 kW and some change. So the average American home in the average American city needs 6.62-kW of solar panels installed.

If you’re following along at home, here’s the calculation so far, so you can plug your own numbers in:

12-month usage ÷ map area number ÷ 0.85 = system size

Now comes the hard part: limitations on system size.

Step 3: Check if your utility offers net metering

It’s all fine and dandy that you know exactly how much solar you need to meet all your energy needs, but solar panels can never provide all your energy at exactly the time you need it. That’s why many decades ago, some very smart people invented this thing called “net metering.”

They invented it, looked upon their creation, and decided it was good. But ever since solar panel prices have fallen so low most homeowners can afford them, utility companies have been fighting against it.

Nut metering in a net shell

Basically, net metering is the idea that your utility company will take any extra solar energy off your hands when your system produces more than you can use at any given moment, then they’ll give you energy from the grid when the sun isn’t shining. They keep track of the kWh you send to them and the kWh they send to you, and net out the difference.

For example, if you overproduce 3,000 kWh on sunny days and consume 4,000 kWh on dark nights, you get billed for 1,000 kWh. Simple, right?

Why utilities hate home solar

Most states still offer net metering, but it’s becoming increasingly threatened. You see, utility companies decided they didn’t want homeowners muscling in on their racket. After all, most electric providers are investor-owned regulated monopolies, and they have an obligation to maximize profits for their shareholders. You and a few hundred other folks in your city taking your little houses out of the equation means they don’t get to make as much profit.

Some states never got around to making net metering the law of the land. Some states have taken net metering away after years of it being policy. And many states are in the process of “studying changes” to net metering, which is done by state Public Utilities Commissions, usually staffed full of people who (you guessed it!) used to work in high levels at investor-owned utilities. That’s where the “regulated monopoly” becomes the “self-regulated monopoly.”

Given the above truth, it pays to know where net metering is available. Here’s a map!

If you live in one of the states with net metering or something like it, it’s very likely cool for you to install enough solar to meet 100% of your needs. Many places even allow systems sized up to 120% of your last 12 months’ usage, so if you plan to get an electric vehicle and charge it with solar, you’re covered.

If you live in a state where some utilities offer net metering, check with your utility company.

If your state doesn’t have net metering

This is where it can get a little touchy, because policies vary widely in these states.

There are a few possible scenarios:

Your electric utility won’t buy back your solar energy at all, meaning if you go over your usage, you get nothing. Nada. Bupkis.

Your electric utility will pay only their “avoided cost rate” (read: wholesale price) for extra electricity, meaning you’ll get 3-4 cents for every extra kWh in a single month, with no rolling credits.

You get “net metering lite” which involves a “feed-in tariff” (price paid to you for solar kWh) less than retail, but higher than wholesale.

In all of these cases, you’ll still be able to connect to the grid and reduce your electric bills with solar. But it is not financially smart to have a full-sized solar system, because there will be months where your production exceeds your usage and you’ll get little if any credit.

Your best bet is to reduce the size of your system so that you’ll never exceed your usage. That might mean 50% of the size or 80% of the size, depending on your state’s specific policies. You know who knows those policies better than ever? Our solar expert partners. Connect with an installer near you to find out the perfect size for your system.

Step 4: Calculate your solar panel needs

Okay, you’ve wended your way through the forest of net metering policy, and you’ve come out on the other side ready to see how many panels you need! Recall from above that our average home in the average U.S. city needs a 6.62-kW system to make the 9,000 kWh it needs every year.

To find out, take system size in kW divided by .320 kW per panel, and round.

We get 6.62 ÷ .320 = 20.69, rounded up to 21.

So the average home in the average place in the United States needs twenty-one 320-watt solar panels. That’s a 6.72-kW system capable of making about 9,100 kWh per year—a bit more than the 9,000 kWh we need, but the difference doesn’t matter that much, and the output of the panels will actually decrease slightly over the years as weather and time take their toll on the panel glass. If you live in a good net metering state, you could round up and overproduce for a few years…

If you’d like to do your own numbers, here’s the final equation:

12-month usage ÷ map area number ÷ 0.85 = system size
system size ÷ kW per panel = number of panels

Step 5: See if the panels will fit on your roof

Oh! Here’s the last wrinkle: will those 21 panels fit on your roof? Assuming you have a nice, un-shaded, south-facing roof (west-facing in a pinch), how much space do you need?

Solar panel setback

When it comes to solar panels, the term setback means a certain amount of space that needs to be clear between the panels and the sides and ridgeline of the roof.

In California, for example, the rule states that a setback of 3 feet is necessary to allow firefighters to access the parts of the roof that are essential to doing their jobs properly. As you can imagine, this eliminates a good deal of available roof space. Most other states follow California’s lead here, because they were the first to lay out these rules.

Here’s how setback looks in practice:

Source: calfiresetbacks.com

That 3-foot space all around the edges of the roof (except the bottom) can make a huge difference. We assume it’ll remove about 25% of total roof space from consideration, leaving 75% of your south- or west-facing roof available for solar panels.

Calculating how many solar panels will fit on your roof

The first thing to account for here is that solar panels can only go on a roof that faces south-ish. That could be southwest or southeast, but due south is best. So when calculating your roof’s area, only consider those portions that face south.

Once you know the area of your south-facing roof, follow these steps to see how many panels will fit:

Take your available square footage and multiply by .75 to account for setback

Now take your 450 square feet and divide by 17.6: 450 ÷ 17.6 = 25 panels would fit on your roof. That’s enough for the average 21-panel, 6.72-kW system!

If you don’t have enough space for your system: Simply multiply the number of panels that will fit, based on the last calculation, by 320 watts per panel, then multiply the result by the number of the area where you live in the map above. That’ll give you how many kWh your smaller system would produce. You could also choose similar-sized panels rated for higher production, but they may be more expensive.

So with 600 square feet of roof space, you’d fit our 6.6-kW system made up of twenty-one 320-watt panels. You can do the calculation yourself or use our handy little solar calculator below to find out!

How much money solar panels can save you

Now this one really depends. The biggest two factors that determine how much money your solar panels save you are incentives and electricity cost. Incentives are tricky, because they vary from state to state (and sometimes from month to month). But electricity cost is easier to figure out, because you get a bill every month with the cost per kilowatt-hour (kWh) right on there!

Now that you know how much electricity your panels can produce in a year, look at your utility company’s website or check your bill for your rate. Be sure to add in the per-kWh charges for transmission and distribution, because you’ll be saving on those too!

Last of all: multiply your electric rate by the number of yearly kWh from the last step. In St. Louis, they pay about $0.12/kWh for electricity. So $0.12/kWh x 9,000 kWh/year = $1,080 saved in one year with our 6.6-kW solar panel system!

connecting into a sub-panel? I have a flat section of my roof that I want to install solar panels on. My house is large, and the amount of power generated will never satisfy the house’s total needs. The flat roof is very close to a sub panel that powers most of the second and third floors, but those floors don’t use much power since the refrigerators/ice machines and geo-thermal chillers are all driven from the main panel. So my question is: Will solar power connected to a sub-panel provide power to the entire house? (or just those circuits on the sub-panel)?

I need help in finding out: (1) how many solar panels do we need for our 4,000 square feet house? (2) how much would it cost? (3) how much can we save? (4) what are the pros and cons of paying for the system upfront vs. leasing it from the company which installs it? How long would the system be good for with little or no maintenance issues. Our zip is 94127. We have a flat roof of 30′ x 50′ approximately. There is no obstruction in all directions. We don’t need any air conditioning. We have two refrigerators, but no freezer at this time. Currently, most of the house is heated by a gas furnace except for an area of about 700 s.f. that needs to be heated with electric heaters. Most of the lighting fixtures are using LED bulbs. The PG&E bill for electricity is around $150 to $200 per month.

The home that i want ot build is about 7,400 sq ft its my plan to use 2 each Tesla power walls and rather than puntting the cells on the roof I will build a large gazebo next to the driveway and install about 50 panels on top of the gazebo then wire directly in the the powerwalls in the garage about 20 ft away, what is frustrating is tryihg to design a system thats totallyt off grid. I will have a emergency propane generator for backup if there is a need to recharge the powerwall I dont see anything online to help with the design

Hey Andre- If you read our Off-Grid page, you’ll get the chance to use the calculators we’ve come up with to design a system that will meet all your needs, including energy storage. Just bear in mind that the amount of electricity used for a 7,400 square foot house is going to require a very large solar array and lots of batteries, probably costing in the range of $50,000-$75,000, even of you buy from a place like the AltE Store or Wholesale solar and install yourself.

Hello, from my calculations my DC size is 121.6 KW with a roof area of 811 m2. So i am trying to calculate the number of panels that can be installed on the roof and i getting roughly 150 panels of 260W which gives me 40KW. WHy is the actual lower than the calculated DC size of 12i.6 kw?

I don’t want them messing with the roof over my head as when the roof leaks the roofer will blame it on the solar installer and the solar installer will blame it on the roofer. So I have 160 sq. ft. roof over the patio facing south that’s available. That comes out to about a $400 a year savings. How much would solar panel, wiring, and inverter cost? If you lose power to the house from PG&E, how hard is it to synch the power waves, frequency and voltage before combining the two sources?

If your in California and your contemplating installing a solar system yourself start with the “California Solar Permitting Guidebook”. Once you calculatalculate in your roof edge set backs and fire pathways you’ll see that your not going to get anywhere close to as many panels as this article suggest. If there’s anything pertaining to set backs and pathways that your still unsure about you can always go to your city or county building & safety department and ask.

There is no natural gas in my area. That leaves electricity for hot water, and and heating. In the winter, I use 4500 KWH per month. Summer I use about 1/3 of that to manage a well pump and pool filter etc. What type of system would be best ? Solar panels, solar evacuated tubes, or ground source heat pump ?

Hey Ben, awesome article. You really broke it down into a science which is super useful. If I wanted to install extra solar panels and tie it to the utility grid, how long do you think it would take for those panels to pay for themselves?

Could you share a table showing approximate current cost per panel, panel manufacturer, respective max watt rating and whether they use micro inverters or a collective inverter, not including installation cost?

Many of the big installers are rapidly switching supply chains to 280 or 295 watt panels as the “standard” offering. Also, in the Southwestern US, a west facing roof can get almost as much production as a South face. In sunny AZ, NM and southern CA you can even use due North face. Not the most optimal but it can pencil out. Do your own calcs but please have a professional installer do a survey. They have access to very accurate sun reading tools and mountains of data regarding sun hours and real world performance that you do not.

The basis for the calculations in this article are very misleading. Big rectangular panels seldom pack in a space the way you would like. For example, my roof is all hips, so from the get-go I am fitting rectangular panels into a triangular space. One face of my roof for example is 200 sq. ft., and while this article claims I could fit 11 panels, I can actually only fit 7. That’s not much more than 1/2 what this article says! Don’t overestimate rectangular roofs either. You might lose and entire row, or column, or both if things don’t align right.

The only accurate way to determine the generation potential of your roof is to actually mock-up the panel layout on your roof using the dimensions of the panels you choose, and only then will you really know how many panels will fit.

Thanks for the reply, Chad. And it’s an excellent point you’re making. This article does assume a rectangular roof, or at least that you can carve rectangular sections out to fit your panels on. Maybe we’ll tackle hips in a future post!

i want to know for a 1kw power how much square feet will be needed how it can be calculated.one more thing for industries how they can use this solar panel for their working process eg:palywood industry

My roof allowing a three foot setback on all sides is 1198 sq. ft. I have a five ton heat pump, 50 gallon hot water heater, fridge,washer/dryer,stove top 4 burner is my roof adequate for my needs ? Thanks !

Switch to solar and save $36.45/mo on average ($0 installations may be available)

Solar Power Rocks has been referenced by:

Solar Power Rocks provides free comprehensive guides to solar policy and incentives for all 50 states and the District of Columbia, along with hundreds of helpful and informative articles about recent solar news and general information related to home solar power. For media inquiries, general questions, or to report an error, you can reach us here.